Presently most extensively used reference electrodes (comparison electrode) such as silver/silver chloride electrodes (Ag/AgCl electrode) or calomel electrodes (Hg/Hg2Cl2) are used to generate a stable potential by immersing an electrode comprising a metal body and a slightly soluble salt film that covers the metal body and that is made of slightly soluble salt to the metal in an aqueous solution comprising an anion constituting the slightly soluble salt such as, for example “chloride salt ion (Cl−), and are referred to herein as reference electrodes based on a second electrode principle. The potential of such a reference electrode is stable, however, since the potential depends on the Cl− concentration in the internal aqueous solution, it is necessary to keep the concentration constant. In addition, a salt bridge is required in order to prevent the internal aqueous solution from directly attaching the sample solution and to keep the liquid junction potential constant.
For the above-mentioned reference electrode, the KCl aqueous solution is used as the internal aqueous solution, and in order to minimize the fluctuation of the liquid junction potential between the internal aqueous solution and the sample solution, the internal electrode comprising Ag/AgCl or Hg/Hg2Cl2 or the like is immersed into the internal aqueous solution comprising a KCl aqueous solution of high concentration (3.3 M (M=mol/dm3)˜saturation) so that the internal aqueous solution makes contact with the sample solution through a junction comprising a porous body such as ceramics or glass. (Refer to, for example, patent document 1).
However, if the KCl solution of high concentration is used as the internal aqueous solution of the reference electrode, there are problems; the sample solution is polluted because the K+ and Cl− move to a side of the sample solution on a constant basis so that KCl elutes out into the sample solution, and there is a need to frequently fill in or exchange the internal aqueous solution because the KCl concentration of the internal aqueous solution decreases. In addition, there is another problem that an accurate measurement is hindered because the AgCl dilutes from the internal electrode and deposits and attaches inside a pore of the junction so that the junction is clogged. Furthermore, for a reference electrode using an internal aqueous solution, there is a problem that it is difficult to downsize the reference electrode because it is necessary to consider the volatility of the internal aqueous solution in designing the reference electrode and, for example, to enlarge a capacity to accommodate the internal aqueous solution.
Meanwhile, there is an attempt to gelatinize the internal aqueous solution or to cover the internal aqueous solution by an electroconductive polymer, however, nothing can be comparable to the reference electrode using the internal aqueous solution in stability and repeatability of potential.
Recently the present inventors invented a salt bridge using slightly soluble ionic liquid. By the use of a reference electrode using the salt bridge, it is possible to almost completely remove the fluctuation of the electric potential difference at the junction, to minimize the pollution in the sample solution, to lessen frequency of filling in and exchanging the internal aqueous solution, to prevent clogging at the junction, to maintain the durability of the reference electrode for a long time and to conduct measurement with high accuracy.
However, since this reference electrode still uses the internal aqueous solution, there is a problem that it is difficult to downsize the reference electrode because it is necessary to consider the volatility of the internal aqueous solution in designing the reference electrode as mentioned above.
In addition, there is a planate reference electrode wherein a surface of an Ag/AgCl electrode is coated with a PVC (polyvinyl chloride) film comprising an ionic liquid as shown in non-patent document 1.
In non-patent document 1, for an ionic liquid impregnated solid PVC electrode, a Cl− ion is an anion for both the ionic liquid and the ion-exchange material and a Nernst response is expected for the Cl− ion in the solution. In addition, generally such a result was shown (refer to non-patent document FIG. 2˜FIG. 5). As a result, since this electrode does not have a function of salt bridge, it is not possible to use the electrode by itself for a pH measurement method using a potentiometer measurement method (potentiometry) wherein a glass electrode is combined with a reference electrode having a salt bridge. In addition, it is not possible to use an electrode by itself also for other potentiometry, and there is a problem that the salt bridge has to be combined.
Patent document 1: Japan Patent Laid-open number 11-258197
Non patent document 1: MAMINSKA Renata, etc. All-solid-state miniaturized planar reference electrodes based on ionic liquids, Sensors and Actuators B, number 115, p. 552-557, May 23, 2006